The present invention relates to a method for the control of an electrical machine, a drive system and a method for determining the mutual position of moving parts in an electrical machine.
Electrical machines have two parts which are movable in relation to each other, commonly called the xe2x80x9cstatorxe2x80x9d resp. the xe2x80x9crotorxe2x80x9d. The most common types of electrical motors have a rotor which is an element which is suspended so that it can rotate inside a stator. The motor is provided with coils which can be provided with electrical current, whereby a magnetic flow is produced. The rotor in combination with the stator form a magnetic circuit through which the magnetic field produced by the coils flows.
When the mutual position between the rotor and the stator changes then the reluctance in the magnetic circuit changes.
In order to drive a reluctance motor with a number of windings the current is connected to the windings in a way which depends upon the position of the rotor in relation to the stator.
A known way of providing this control of the current includes sensing the position of the rotor with the help of separate position sensors coupled to the rotor, whereby the position sensors produce an output signal which depends on the position of the rotor.
Another known way of achieving control of phase currents uses the fact that the inductance of a phase varies depending on the position of the rotor in relation to the stator. The patent publication PCT/SE87/00442 describes a method of determining the position of the rotor for a reluctance machine starting from the equation
(Uxe2x88x92Rxc2x7i)=d/dt(Lxc2x7i), 
where i is the current through the phase windings, where U is the voltage across a series connection of the phase winding, a transistor valve and a current sensor resistor, and where R is a predetermined constant corresponding to the sum of the resistances in a phase winding, an activated transistor and a current measuring resistor.
WO 97/11524 discloses a method for estimating the rotor position of a switched reluctance motor by estimating a phase flux from at least one conducting phase, estimating an inductance in at least one non-excited phase, sampling phase currents and combining the estimated phase flux with the estimated inductance.
The present invention relates to the problem of obtaining an improved determination of the mutual position of movable parts in a reluctance machine.
This above mentioned problem is addressed by a method comprising the steps of:
generating a signal corresponding to a current through a phase winding;
generating a signal corresponding to a voltage across the phase winding;
producing an amplitude signal value depending on the current signal, the voltage signal and a parameter value, the parameter value substantially corresponding to a resistive loss in the phase winding; and
establishing a position value depending on the amplitude signal value. The method further includes the step of generating the parameter value in accordance with a relation between the current signal and the amplitude signal.
The invention further relates to the problem of providing improved regulation of the torque of an electrical machine without any separate sensor for detecting the position of the rotor by means of the shaft of the motor.
More precisely, the invention relates to the problem of producing improved regulation of the torque at both high and low rotation speeds without separate rotor position sensors.
Further, the invention relates to the problem of providing control of the torque of a reluctance motor with the use of estimations of phase winding values such as the reluctance values or magnetic flow, in order to determine the position of the rotor. The invention also concerns the control of the torque of the motor with the help of estimated phase winding values, wherein the phase winding values are estimated with improved accuracy and with reduced or completely eliminated dependence on the speed of rotation of the motor and the temperature of the motor windings.
A reluctance machine comprises two mutually movable parts and at least one phase winding which has a resistance and the inductance of which depends on the mutual position of the parts. A device for regulating a reluctance machine comprises a controllable valve, which is connected in series with the phase windings and which is adjustable between as essentially open state and a conducting state. The above mentioned problems are solved with a method for controlling of the valve comprising the steps of:
a) measuring a current through the phase winding;
b) measuring a voltage across the phase winding;
c) producing a signal value depending on the measured current value and the measured voltage value in accordance with a mathematical expression comprising an adjustable parameter;
d) determining a relationship value between the signal value and the measured current value;
e) adjusting the parameter value depending on the relationship value.
The invention uses the fact that the magnetic flow is in phase with the winding current for a machine of the above described type, in order to estimate the magnetic flow depending on a measured current value, and in order to with increased accuracy control the electrical machine.
According to one embodiment a signal value is produced corresponding to the magnetic flow "psgr" in accordance with the mathematical expression
"psgr"=∫ULWdxcfx84
where
ULW=K3*Ud+iWxc2x7K1+K2 
The adjustable parameter value K1 relates to the resistance in the phase winding, the parameter K2 corresponds to a voltage drop in the valve, and the parameter K3 depends on the actual working state of the machine.
The above described solution gives the advantage that the estimate automatically adapts the control parameter K1 so that the magnetic flow estimate has a good correspondence with the true magnetic flow even when the winding of the motor changes its series resistance RW, e.g. as a consequence of temperature changes. If the voltage Ud is determined over both the phase winding and the valve then the parameter K1 is also adapted with reference to possible resistance changes in the valve.